In 2009, standardization of 10G-EPON (10 Gigabit Ethernet Passive Optical Network: “Ethernet” is a registered trademark) in IEEE802.3av was completed. The 10G-EPON features transfer 10 times faster than GE-PON (Gigabit Ethernet Passive Optical Network: see non-patent literature 1) already in widespread use.
An overview of the arrangement of a GE-PON system related to FIG. 13 will be described. In this GE-PON system, an OLT 100 transfers frames between a host apparatus (not shown) and a plurality of ONUs 3 connected via an optical splitter 2.
The OLT 100 for the GE-PON incorporates an optical transceiver 11 and a PON control circuit 12. In the OLT 100, the optical transceiver 11 performs electro-optical conversion of a downstream frame (downstream electrical signal DS) into an optical signal to be output to the ONUs 3 connected via the optical splitter 2, and photoelectric conversion of an upstream frame (optical signal) from one of the ONUs 3 into an electrical signal (upstream electrical signal US).
The IEEE standard defines that the maximum number of ONUs 3 connectable to one optical transceiver 11 in the OLT 100 is 32. Thus, if it is necessary to connect 33 or more ONUs 3 as stations accommodating the ONUs 3, in general, a plurality of optical splitters 2 are provided between the OLT 100 and the ONUs 3 and a plurality of optical transceivers 11 and a plurality of PON control circuits 12 are used, as shown in FIG. 14.
In the 10G-EPON system as well, the IEEE standard defines that the maximum number of ONUs connectable to one optical transceiver is 32. Since, however, a PON control apparatus for 10G-EPON is required to have performance (10-time data transfer rate) higher than a PON control apparatus for GE-PON, the cost of the apparatus (the purchase price of the apparatus or the like) is higher. Therefore, as a problem for adopting the 10G-EPON system, the system cost (connection cost) for one ONU needs to be made as low as possible.
As a countermeasure against the above problem, it is considered to reduce the number of optical transceivers and that of PON control circuits for one ONU by increasing the number of ONUs connectable to one optical transceiver. For example, there is proposed a technique of allowing connection of 33 or more ONUs by using an optical amplifier (see, for example, patent literature 1).
However, there is still a problem that the cost (purchase cost or the like) of the optical amplifier is higher than that of a part (LSI or the like) for an electrical circuit.